SHOWS FOR YOUNGSTERS AND THEIR PARENTS TOO : Larisa Oleynik finds the secret is to have fun as 'Alex Mack'

The last thing your average grunge-loving teen-ager wants is to get all covered with icky stuff. That would ruin the carefully put together "I-don't-care-how-I-look" look.

In the new Nickelodeon series "The Secret World of Alex Mack," 13-year-old Alex (Larisa Oleynik) is less than thrilled when a tank of ooze slimes all over her when she's on the way home from school. Then Alex discovers the icky stuff is actually an odd chemical that gives her superpowers--she can morph into a puddle of liquid and she can will items to come to her, among other things.

She's like a teen-age "Bewitched." And in the tradition of the nose-twitching Samantha, Alex uses her powers to just get through life, not catch criminals or save the world.

Nickelodeon hopes the series fills the void on Saturday night's Snick block that was left by the popular "Clarissa Explains It All," no longer in production.

San Francisco Bay area-native Larisa, who is the same age as the teen-ager she plays, may have wished for some superpowers a few days before meeting a reporter in Santa Monica. The eighth-grader arrives perkier than her TV counterpart, but with a bold purple shiner under her left eye.

Slightly embarrassed, she giggles through her explanation: "I was over at a friend's house, and he has a big trampoline. I was practicing my flips and I landed really, really hard on my bottom and my head hit my knee. Hard."

She picks up her egg-salad sandwich and smiles. "I didn't have any idea it would be this bad," she says apologetically. She's assured it doesn't look that bad.

Wearing a plaid jumper, Larisa looks the picture of the private school student she is--quite a switch from Alex's grungy garb.

Larisa's also more serious about school than Alex. Lest there be no misunderstanding about why she's in L.A. on a school day, she quickly explains that even though she's flown in from San Francisco to promote Alex Mack, she attended school in the morning--including her hardest class, grammar.

But she's just as quick to launch into how much fun she's had being Alex. "She's like me in some ways," she says. "She has a crush on this boy ... and every girl has those. She has a best friend and she has a hard time deciding things."

The 13 episodes were shot in Valencia in August, and Larisa was thrilled to become friends with actress Meridith Bishop, who plays Alex's older sister, Annie.

"She's an only child like me!" she says. "And we got really close. I just love her."

An animated Larisa swings into high gear when she discusses the show's last episode.

"I got to fall into a mountain and down a mine shaft!" she says. "It was great!"

She acknowledges, however, that most of the tough stuff was done by a stunt double. Does she think her stunt double looks like her?

"No!" she answers emphatically. "God, she was really old. "

"She's exactly my age," her mother, Lorraine, responds dryly, but with a smile.

"Well, in comparison to Larisa, she's old," chimes in a Nickelodeon representative.

Larisa's mother smiles.

Oleynik indulged her daughter--then 8--when Larisa begged to audition for the San Francisco production of "Les Miserables." She got the role. After a year on stage, Larisa was hooked. Each year after that the Oleyniks summered--during pilot season--in Los Angeles. In the summer of '93, Oleynik realized how serious--and good--Larisa was when she won the role of Alex Mack over 300 other girls. "It was a nice surprise," the proud mother says.

"Larisa had such a great natural instinct. It was stunning," adds creator and producer Tom Lynch, joining the table. "She made the character richer and more shaded." He says he didn't want a girl who "looked like she worked too much in television, had the kinds of reactions you always see, the smile, the look, the acting with eyes and all that kind of thing. Larisa was just perfect."

Which is a good thing for Frente!, Stone Temple Pilots and Cranberries lover Larisa--since she's now completely motivated to continue to act: "You get to be a different person each time. I really want to do more stuff. Movies. A movie would be great. Neat."

"The Secret Life of Alex Mack" premieres on the Snick block Saturday at 8 p.m. on Nickelodeon. For ages 5 and up.